how to align and array
In Editmode, select the face and choose View >> Align View >> Align View to Active. 'Top' usually works. 

Next add a cube and put the checkmark on 'Align to View' in the redo Panel to the left ( or F6). Back in medieval days, objects were aligned to view by default.

Now you can copy the cube's rotation for your stone.

By the way: the original wall in the picture is vertical, no need to align anything.